All man-made structures are subject to stresses and strains during the course of their existence which directly affect the structural integrity of the object. In time, the accumulation of stress, strain and vibration can result in degradation of materials making up the structure and the points of attachment between materials. In the case of a transmission tower, the force of wind against the structure as well as ground transmitted vibration can degrade the structural integrity of the transmission tower. Similar types of stresses and vibration, and in particular earthquake vibration, can degrade multistory buildings. One particular structure which presents a continuing need for verification of structural integrity is the bridge or freeway overpass. These structures are continually subjected to repetitive loading, often in excess of limits, as well as both ground and traffic vibration. This results in deterioration of the bridge structure eventually requiring repair or replacement of the entire bridge itself.
Presently, however, determinations of structural integrity are generally estimated by assuming parameters of loading frequency and maximum loading weight in order to make an estimate of bridge life. These estimates combined with visual inspection have been utilized to determine when bridge replacement should occur or when a reduction in the posted weight limits of a bridge or highway overpass should be made.
For both safety and economy, it is desirable to accurately determine the structural integrity of a transmission tower, building or bridge. In the particular case of bridges and highway overpasses, it is necessary for state and federal governmental entities to detect when bridge replacement should occur and to be able to forecast such replacements to avoid injuries and for proper financial management. This task is complicated by the fact that knowledge of the actual stresses to which any individual bridge has been subjected is largely unknown.
While estimations of traffic loading and stress and inspections of bridge exteriors can be made, structural degradation cannot be accurately predicted or determined as no means exist for documenting the stress history of a particular structure. Such loading history would allow engineers to determine whether a particular structure had been subjected to a greater or lesser degree of loading than estimated. This information would assist the ability to forecast bridge replacement in two ways-first, in making an early determination of which bridges are in fact structurally impaired, but not as yet exhibiting visible signs of degradation; and second, in determining which bridges may be selected for extended service due to fewer and lighter loading since their construction.
Therefore, it is an object of the present invention to provide a means for documenting the loading and vibrational stress applied to a structure during the lifetime of the structure;
Another object of the present invention is to allow comparisons to be made over the lifetime of a structure with baseline data on the particular structure in order to determine when sufficient structural degradation has occurred to require replacement;
Another object of the present invention is to provide a means of identifying which bridges and other structures may receive an extended useful life and to distinguish those structures from structures subject to overloading and a reduced structural lifetime; and
Yet another object of the present invention is to provide a means of forecasting the need to replace bridges and other structures through accumulation and comparison of structural stress and loading data on individual structures.
Other objects and advantages of this invention will become apparent from the following description taken in connection with the accompanying drawings, wherein is set forth by way of illustration and example, an embodiment of this invention.